As a conventional ink for inkjet printing, generally, an ink obtained by dissolving any of a variety of water-soluble dyes in an aqueous medium is used. Recently, an ink obtained by dispersing a pigment in an aqueous medium is also proposed. This is because the ink using a pigment is excellent in water resistance and light resistance as compared with the ink using a water-soluble dye.
When an inkjet ink is prepared by dispersing a pigment in a high volatile solvent such as water, the ejection performance from an inkjet head must be taken into consideration. When the solvent component volatilizes, the balance of the blending ratio of the pigment component to the solvent component is lost, and the concentration of the pigment in the ink in the vicinity of a nozzle extremely increases. As a result, a phenomenon such as an increase in viscosity of the ink or deposition of a coarse particle is caused. The nozzle is clogged, resulting in deterioration of the ejection performance. Specifically, a predetermined ejection volume is not obtained, and the flying direction turns out to be inaccurate. In some cases, a misfiring nozzle appears, too.
In many conventional pigment inks, by mainly suppressing penetrability, wetting of the ink against the paper surface is suppressed, thereby retaining ink droplets in the vicinity of the paper surface to secure the print quality. However, such type of ink cannot always be adapted to all types of paper and a difference in bleeding due to a difference of the paper type is large. In particular, when recycled paper is used, a stable print quality cannot be obtained. The recycled paper is produced by mixing a large number of unspecified types of paper. A difference in wetting property of the ink against the type of used paper affects the occurrence of bleeding. Also, when a conventional pigment ink is used, it takes a long period of time to dry a printed area due to the composition thereof. In particular, in multicolor printing such as color printing, colors printed side by side are mixed with each other. Further, since the pigment remains on the surface of paper or the like, there is also a problem that the abrasion property is deteriorated.
When a solvent having high moisture-retaining ability such as glycerol is used, although the ejection performance can be secured, the moisture-retaining effect of such a solvent is maintained even after the ink penetrates into paper. When a large amount of such a solvent is used, paper is deformed, for example, paper wrinkles or curls, and therefore, it is also necessary to limit the content of such a solvent.
In order to solve such problems, an inkjet ink which contains a solvent composed of water and an oil component, that is, an emulsion type inkjet ink is proposed.
The ink described in U.S. Pat. No. 5,531,816 is a pigment-based ink in the form of a microemulsion obtained by using a water-insoluble organic compound and a hydrotropic amphiphile. Since an ionic compound is used as the hydrotropic amphiphile, the pigment dispersion stability was affected in some cases. There is a concern that this may lead to deterioration of ejection performance.
The emulsion ink described in JP-A-2006-56931 contains an oil component having an initial boiling point of 290° C. or higher and a nonionic surfactant having an HLB of 3 to 5. The ink is a water-in-oil (W/O) emulsion and as the oil component, a low volatile mineral oil, synthetic oil, vegetable oil or the like is used. The properties such as clogging resistance and ejection stability are improved, however, print quality in terms of strike-through is not improved. This is because the penetrability into paper is increased. Further, since most of the oil component impregnated into paper does not volatilize, a method for storing a printed matter formed using such an ink is limited. When a printed matter is stored in a transparent resin film sheet, the film sheet swells and softens due to the action of the oil component and therefore turns out to be unusable. JP-A-2005-54141 describes the ink in which a saturated hydrocarbon solvent with high hydrophobicity having a boiling point of 180° C. or higher is used as the oil component, which has a similar problem.
As described above, the current situation is that an ink which is an ink for inkjet printing suitable for printing on a paper medium, is excellent in quick-drying property and image quality of a printed matter and does not cause clogging of the ink in intermittent ejection and is capable of stably dispersing a pigment over time is not obtained yet.